Nickel salt solutions are very useful for a number of applications, particularly electroplating and production of nickel hydroxide. While electroplating has been a main use for nickel salt solutions, the production of nickel hydroxide has become an increasingly popular application for nickel salt solutions, particularly nickel sulfate solution, due to the growing demand for batteries utilizing nickel hydroxide as an active material. Two of the main types of batteries that utilize nickel hydroxide as an active material are the Ni—Cd (nickel cadmium) type and the Ni-MH (nickel metal hydride) type. In both Ni-MH and Ni—Cd batteries the positive electrodes are made primarily of nickel hydroxide active material.
Ni-MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in an alkaline electrolyte.
Upon application of an electrical current across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the absorption of hydrogen formed by electrochemical water discharge reaction and the electrochemical generation of hydroxyl ions. The negative electrode reactions are reversible. When the Ni-MH cell is connected to a load, the metal hydride active material of the negative electrode is discharged thereby releasing the stored hydrogen to form water and release electrons. The charging/discharging process for the negative electrode of a Ni-MH cell is governed by the following reversible reaction:

Upon application of an electrical current across a Ni-MH cell, the Ni (OH)2 material of the positive electrode is charged by the formation of NiOOH from the Ni (OH)2, the electrochemical generation of water, and the release of electrons. The positive electrode reactions are reversible. When the Ni-MH cell is connected to a load, the active material of the positive electrode is discharged thereby forming Ni (OH)2 and electrochemically generating hydroxyl ions. The charging/discharging process for a nickel hydroxide positive electrode in an alkaline electrochemical cell is governed by the following reversible reaction:

The use of nickel hydroxide, Ni(OH)2, as a positive electrode material for batteries is generally known. See for example, U.S. Pat. No. 5,523,182, issued Jun. 4, 1996 to Ovshinsky et al., entitled “Enhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cells”, the disclosure which is hereby incorporated herein by reference.
Several forms of positive electrodes presently exist and include, but are not limited to, sintered, foamed, and pasted electrode types. Processes for making positive electrodes are generally known in the art, see for example U.S. Pat. No. 5,344,728 issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference. The particular process used can have a significant impact on an electrode's performance. For example, conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. Sintered positive electrodes are constructed by applying nickel powder slurry to a nickel-plated, steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact, resulting in a porous material that is approximately 80% open volume and 20% solid metal. The sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by the conversion to nickel hydroxide by reaction with an alkali metal hydroxide. After impregnation, the material is then subjected to electrochemical formation.
To achieve significantly higher loading, the current trend has been moving away from sintered positive electrodes and moving toward pasted electrodes. Pasted electrodes generally comprise nickel hydroxide particles in contact with a conductive network or substrate, most commonly foam nickel. Several variants of these electrodes exist and include plastic-bonded nickel electrodes, which utilize graphite as a microconductor, and pasted nickel fiber electrodes, which utilize spherical nickel hydroxide particles loaded onto a high porosity, conductive nickel fiber or nickel foam support.
The production of low cost, high capacity nickel hydroxide is critical to the future commercialization of Ni-MH batteries. As with electrode formation, the properties of nickel hydroxide also differ widely depending upon the production method used. Generally, nickel hydroxide is produced using a precipitation method in which a nickel salt solution, such as nickel sulfate solution and a hydroxide salt are mixed together resulting in the precipitation of nickel hydroxide.
It has been discovered that nickel hydroxide suitable for use in a battery electrode should have an apparent density of 1.4-1.7 g/cm3, a tap density of about 1.8-2.3 g/cm3, and a size range of about 5-50 μm. Active, nickel hydroxide particles are preferably spherical in shape with a high packing density and a narrow size distribution. Preferably, average particle size should be about 10 μm and tap density should be about 2.2 g/cm3. Paste made with nickel hydroxide having the aforementioned properties has good fluidity and uniformity, and thus allow the fabrication of high capacity, uniformly loaded electrodes. The use of this kind of nickel hydroxide also improves the utilization of the active material and the discharge capacity of the electrode. If the process for making nickel hydroxide is not carefully controlled, the precipitated nickel hydroxide will have an irregular shape and/or low tap density. For example, if the rate of reaction is too fast, the precipitate formed may be too fine resulting in a low tap density. A fine powder having a low tap density requires longer filtering or washing times and increases the adsorption of water on the surface of the nickel hydroxide particles. Furthermore, if the precipitated nickel hydroxide particles have too wide a size distribution (ranging from 1 to hundreds of microns), the nickel hydroxide may require pulverization to render it useful. Electrodes formed with nickel hydroxide having a low tap density may also lack high capacity and high energy density. For these reasons and others, an active powder having an irregular shape and/or low tap density is less desirable for use as a high capacity battery electrode material.
In order to produce substantially spherical nickel hydroxide having a high tap density, particles are gradually grown under carefully controlled process conditions. A nickel salt solution is combined with an ammonium ion. In solution, the nickel salt forms complex ions with ammonia. When caustic is added, nickel hydroxide is then gradually precipitated by decomposition of the nickel ammonium complex. The reaction rate is difficult to control, so methods have been introduced to separate critical steps in the production process to compensate for said difficulties. For example, U.S. Pat. No. 5,498,403, entitled “Method for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteries”, issued to Shin on Mar. 12, 1996, the disclosure of which is herein incorporated by reference, discloses a method of preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate solution is mixed with ammonium hydroxide in an isolated amine reactor to form a nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. Such a method relies heavily on a raw material sources of very high purity or what is termed throughout the ensuing specification as primary nickel.
Thus, particular notice should be taken in the fact that the current process used widely throughout the industry for making positive electrode materials, such as those described above, have utilized expensive, high grade, and highly pure primary nickel for the production of nickel salt starter solutions. As modern process technology and automation have reduced the cost of labor in the production of nickel hydroxide, the cost of primary nickel and its associated salts have become a significant factor in determining the cost of nickel hydroxide as used for active electrode materials, accounting for up to 60% of the direct manufacturing cost of the final nickel hydroxide.
Primary nickel used for the production of active materials is typically derived from the ores of nickel sulfide and nickel oxide and purified by electro-processing. Nickel sulfide ores are refined by flotation and roasting to nickel oxide. Nickel oxide ores are typically refined by hydrometallurgical refining, such as leaching with ammonia. Refined nickel ore is usually cast into nickel anodes for distribution as primary nickel. The highly pure, primary nickel may then be dissolved into solution, such as a sulfate solution, and sold as highly pure aqueous nickel sulfate, with a frequent end use also being nickel electroplating and electroless nickel plating.
The average amount of nickel estimated to be present in the earth's crust is only about 0.0084 wt %, as reported on page 14-14 of the Handbook of Chemistry and Physics, 78th Edition, 1997-1998. Because nickel is used for many things, including the production of stainless steel, the demand for nickel is very high, making it a relatively expensive metal. Although primary nickel is a commodity product, it is subject to wild market swings in price. For example, during the period of Jun. 1, 1999 through Jun. 1, 2000, nickel prices have seen dramatic volatility having a low of $2.16/lb and a high of $4.77/lb as reported on the London Metal Exchange. As a means of off-setting or hedging against the increasing cost of nickel, a number of large producers of nickel hydroxide have gone so far as to purchase ownership interests in nickel mines. Smaller manufactures of nickel hydroxide, unable to offset rising nickel prices, have been left at a competitive disadvantage.
Current processes for the production of aqueous nickel sulfate (NiSO4) involve dissolving nickel powder in sulfuric acid (H2SO4) which produces nickel sulfate liquid and hydrogen gas. Such processes for the production of nickel sulfate are governed by the following reaction:Ni+H2SO4->NiSO4+H2  (3)This process, however, must be conducted in a very secure environment, due to the volatility of hydrogen gas, which creates a hazardous environment. Additionally, nickel powder (particles less than 0.1 mm) is expensive when compared to bulk nickel (particles greater then 0.1 mm).
A similar reaction may be used to produce copper sulfate. U.S. Pat. No. 6,294,146 to Benet discloses a continuous chemical reaction for producing copper sulfate crystals including a filtering step for separating the copper sulfate crystals from the aqueous reaction medium. During the production of copper sulfate, oxygen and sulfuric acid are reacted with copper whereby sulfuric acid is the limiting reagent. Benet also discloses a process for removing precious metals from base metal alloys where the precious metals are dissolved into solution and the remaining metals such as Ni and Cu remain in the filtrate. While Benet teach methods for producing copper sulfate crystals, there is no teaching of producing salts including metals other than copper. Benet also does not teach the production of metal salt solutions, particularly nickel sulfate solutions for which there is an increasing demand in industry.
While there are presently many known processes for producing metal salts, there still remains a need in the art for improved processes for producing salt solutions, particularly nickel sulfate solution as used for electroplating or nickel hydroxide production. Furthermore, there exists a need for a safe cost effective process for making nickel sulfate solution from nickel, wherein hydrogen gas is not liberated into the atmosphere as a reaction byproduct.